In the manufacture of cam shafts for internal combustion engines, the cam shafts have been made by casting, or by steel forgings or by machining steel bar stock. The cam shafts have a very complex shape. Forging is a difficult process for making complex shapes to close tolerances. Accordingly, the forged cam shafts require some machining. Both of the forging and machining processes are quite expensive and difficult.
Recently, a cam shaft has been made by casting a heat-treatable gray cast iron with the following composition:
______________________________________ Preferred Range General Range Analysis Percent Percent ______________________________________ Carbon 3.25-3.45 3.00-3.60 Silicon 2.25-2.45 1.75-2.60 Manganese 0.60-0.90 .50-.90 Chromium 1.30-1.50 1.30-1.70 Nickel plus copper 0.40-0.60 .40-.80 Molybdenum 0.40-0.50 .30-.75 Sulfur 0.15-max .15 Phosphorus 0.15-max .15 Vanadium 0.25-0.40 .25-.60 ______________________________________
This alloy in as-cast condition has significant carbides and a high hardness, pearlitic matrix. It is necessary to machine these castings somewhat to maintain tolerances. Thus, the castings must be annealed prior to machining. The as-cast hardness of the cam shaft was in the range of 331-364 Brinell. These cam shafts were heat treated by raising the temperature to 1420.degree. F. (771.degree. C.) in four and one-half hours, holding that temperature for four to four and a half hours and then cooling slowly to atmospheric temperature in the oven. The cooling process typically took in excess of six or seven hours.
Although the annealing decreased the Brinell hardness while retaining the carbides, it was found that the machinability was highly irregular and generally unsatisfactory. Some cam shafts thus heat-treated were virtually unmachinable.